Night vision technology has enabled a wide variety of military missions to be executed effectively in light conditions as low as overcast starlight. Digitally enhanced vision technology has the potential to further increase this tactical advantage. The image intensifier tubes used in present direct-view night vision goggles achieve good low light sensitivity, but are physically large and have limitations in image quality, dynamic range, and suitability for system integration that may be overcome with digital technology.
A solid-state digital image sensor with sensitivity for night vision operation may have a dramatic impact on the availability and effectiveness of a next-generation night vision system. High performance uncooled solid-state low light image sensing has been an elusive goal for years.
It is generally recognized that low light image sensing is greatly affected by noise. The reason for this is that in the lowest light conditions contemplated for night vision applications such as overcast starlight, the number of discrete photons arriving at each pixel in a video image sensor during the frame integration time may be very low—on the order of 3-5. With such a small signal, even a very low noise floor will represent a significant portion of the output of the image sensor. For this reason, it is common to cool low light sensors to reduce the magnitude of thermal noise, which is proportional to temperature. While cooling may be an effective way to improve sensor performance, it requires a significant amount of power that makes operation from batteries difficult.
Low light image sensors using avalanche photodiodes coupled with an analog comparator may be used to detect individual photons in the visible to near infra-red spectrum. The avalanche photodiode is biased to operate in a linear gain mode. When a photon arrives, a photo-electron may be generated. The photo-electron is directed by a bias voltage toward the avalanche photodiode junction, where it is accelerated by the high static electric field in the junction. The high velocity electron collides with atoms in the junction region, causing impact ionization action that generates a burst of approximately 100-200 additional electrons. This burst of electrons is accumulated in the capacitive charge storage of a signal node and is detected by an analog comparator, whose output is coupled to a digital counter circuit. Each pixel in the image sensor may be provided with a comparator and counter, which serves the dual functions of effective analog to digital conversion and signal integration.
Even with the electron gain provided by the avalanche photodiode, the signal associated with a burst of 100-200 electrons is quite small. It is quite likely that noise present in the system will cause counts to appear when no photon has arrived, or conversely, to fail to cause a count when a photon has arrived. In either case, the quality of the image will be degraded. What is needed is a method to restore the reduced quality of the images captured by an extremely sensitive but relatively noisy image sensor to generate a high quality images.